Presently, a major portion of point-to-point data communication between data terminal equipment ("DTEs" or terminals), such as personal computers and workstations, is implemented using data communication devices ("DCDs"), such as modems, over analog communications systems, such as the public switched telephone network ("PSTN"). With increasing demand for data transmission, such as with the advent of videoconferencing, there has been a corresponding demand for greater data transmission capability and greater data transmission bandwidth. With this demand for greater data bandwidth, typically unavailable over the PSTN, there has been an increase in use of digital communications networks for voice and data transmission, such as use of the Integrated Services Digital Network ("ISDN"), which have greater data bandwidth capability. ISDN has evolved to provide for end-to-end digital connectivity, and details of ISDN service are described in standards such as the I-Series Recommendations of the International Telecommunications Union ("ITU").
An ISDN Basic Rate Interface ("BRI") service currently available to end users consists of three time division multiplexed channels, comprising two B channels operating at 64 kbps (kilobits per second), which may be used for voice or data, and one D channel operating at 16 kbps, which may be used for packetized data and/or control information (i.e., call set-up and shut down), with additional bandwidth utilized for framing, synchronization, and other overhead bits. An apparatus such as an ISDN terminal adapter ("TA") may be used for transferring data between DTEs (such as personal computers, terminals, local area networks, video conferencing equipment and similar equipment) using BRI service. While two B (data) channels (each operating at 64 kbps) are sufficient for many applications, there are applications in which one data channel having a higher data bandwidth is preferable. As a consequence, a process known as "bonding" (Bandwidth ON Demand) has evolved, in which two or more channels (each having lesser bandwidth) are combined into one channel having greater bandwidth. For example, it may be desirable to combine the two, 64 kbps B channels to obtain a single bonded channel operating at 128 kbps. The 128 kbps channel formed by bonding may then be used to transfer data between two DTEs.
A local terminal adapter and a remote terminal adapter having bonding capability in accordance with an ITU Recommendation, Interoperability Requirements for N.times.56/64 kbit/s Calls, Version 1.0, Sep. 1, 1992, by the Bandwidth ON Demand Interoperability Group, may be used with BRI service to obtain a bonded connection. The terminal adapters may be comprised of a variety of electrical and electronic components and circuits, including one or more integrated multiprotocol processors ("IMPs") (such as a Motorola MC68302), and an interface transceiver (such as a Motorola MC145572 ISDN U-Interface Transceiver). The IMP may have a processor and one or more serial communications controllers ("SCCs") to transfer serial data to and from the processor. The processor utilizes processor formatted data, such as bytes of data, which may be available for processing on a parallel data bus. Typically, however, serial data is transferred between the IMP and the various DTEs, the interface transceivers, and other equipment. As a consequence, in a typical terminal adapter having bonding capability, four serial data streams require SCC servicing (serial to parallel conversion and parallel to serial conversion): the DTE data, each of the two B channels, and the D channel.
One prior art apparatus and method to provide for the four data stream conversions takes advantage of a resource sharing feature of an IMP, such as the Motorola MC68302, which has several SCCS. Because each MC68302 has three SCCs, a first IMP and a second IMP may be coupled or combined to provide six SCCs which are available to process the data streams. Although there are two extra or unused SCCs of the second IMP, the combined IMPs may have greater processing power for some applications, as the memory and other elements of a second IMP may be utilized. While this methodology provides for the four data stream conversions needed for a terminal adapter to have bonding capability, the addition of a second IMP is expensive, requires additional space and connections on a printed circuit board, and adds to the overall cost to manufacture and purchase a terminal adapter.
A second prior art method to provide for the four data stream conversions has been suggested in Appendix D of the MC68302 Users Manual, and may require only one MC68302 IMP. As alluded to in the Users Manual, this method would allow using one SCC to service both B channels, and would rely on synchronizing an enable transmitter command with a frame timing pulse. Although the Users Manual does not suggest or describe how to coordinate such synchronization, those skilled in the art may consider sending frame timing pulses to an interrupt pin on the processor (IMP) to initiate a timing routine. By starting the timing routine with such an interrupt signal, data bytes from a B channel possibly could be inserted at a predetermined time in a framing structure utilized by an SCC. Athough this potential method may have merit, interrupt pins may be required for higher priority features of the terminal adapter and therefore may be unavailable for use in bonding the B channels. In addition, additional processor code and time would be required to implement such a timing routine, and these additional resources may also be unavailable. This possible method may also be prone to difficulties resulting from potential interrupt latencies.
Accordingly, a need has remained to provide SCC servicing of four serial data streams for an ISDN bonded connection, while simultaneously reducing the number of SCCs needed to accomplish the ISDN bonded connection. In addition, a need has remained to accomplish this reduction in SCC requirements with a minimal complexity and expense, while also preferrably using currently available IMPs.